Return Promise for helper.load

[JbIPS]

Hi,

I noticed that helper.load only works with a callback but ultimately relies on a Promise.

Could you return it instead of forcing a callback and no return value?

I can do the PR if it's OK with you.

[knolleary]

A PR would be welcome as long as the callback is still called if provided to maintain the existing behaviour.

[JbIPS]

PR is done, let me know if anything's missing.